1. Fields of the Invention
The present invention is directed to an inverter device for converting an AC line voltage into a high frequency output AC voltage, more particularly to an improved inverter device capable of successfully varying an output energy in a simple control scheme.
2. Description of the Prior Art
A most closest prior art inverter device is disclosed in U.S. Pat. No. 4,564,897 issued on Jan. 14, 1986 which suggests to use one switching element in common to a smoothing circuit of obtaining a smoothed DC voltage and to an inverter switching thus obtained smoothed DC voltage to provide a high frequency AC voltage or current to a load. As shown in FIG. 1 depicting the prior art inverter device, the smoothing circuit 40' includes a smoothing capacitor 41', a choke coil 42' and a first switching transistor 51' which is connected in series with choke coil 42' and which acts to interrupt a pulsating DC voltage obtained through a full-wave rectifier 30' from an AC line voltage to supply choke coil 42' with electromagnetic energy when it is turned on. The electromagnetic energy is released at subsequent turn-Off of first transistor 51' to flow a resulting current into smoothing capacitor 41' for accumulating thereat the smoothed DC voltage. The inverter 50' includes the first switching transistor t1' common to the smoothing circuit 30', a second switching transistor 52,, a transformer 53' with a primary winding 54' and a secondary winding 55', and first and second capacitors 56' and 57'. First and second transistors 51' and 52' are connected in series across smoothing capacitor 41' and are driven to alternately turn on and off for switching the smoothed DC voltage at a high frequency so as to provide a high frequency AC current through primary winding 53', whereby providing a corresponding AC voltage for energizing a load 10' coupled through secondary winding 55' to primary winding 54'. Each of the first and second transistors 51' and 52' is cooperative with primary winding 54' and each one of first and second capacitors 56' and 57' to define a series oscillation circuit which produces an oscillating current allowed to flow in the oscillation circuit through each one of first and second diodes 55' and 59' connected antiparallel with first and second transistors 51' and 52' immediately upon turn off of first and second transistors 51' and 52'. The transformer 53' additionally includes first and second feedback windings 61' and 62' respectively connected to bases of first and second transistors 51' and 2' for self-excitation thereof by the oscillating current at such a timing that one of the switching transistors 51' and 52' is turned on after the other is turned off. In other words, one of the first and second transistors 51' and 52' will not turn on until the other transistor turns off to avoid simultaneous turn-on of the two switching transistors which would otherwise cause dangerous short-circuiting of the inverter device. Even with the advantageous features of the above prior art inverter device that the smoothing circuit and the inverter can share the first switching element and that the short-circuit hazard can be avoided, the prior art inverter device suffers from a critical problem resulting from the common use of the switching transistor. The problem is that it is practically difficult to regulate the inverter output at a desired level stably against a possible input AC voltage variation or in conformity with varying load requirements. For example, when the input AC voltage increases substantially, the smoothing circuit responds to accumulate a correspondingly increased smoothed DC voltage at the smoothing capacitor, which eventually results in an increased load current flowing through the common switching transistor for an extended time interval. The result is that the common switching transistor is kept turn on for such extended time interval, which in turn causes the smoothing circuit to draw more amount of current or energy from the rectifier, thereby further increasing the resulting smoothed DC voltage at the smoothing capacitor and accordingly the load current or energy to a further increased level. Also, when the input AC voltage drops substantially, the common switching transistor will act to ever reduce the load current in a manner opposite to the above, eventually reducing the load current to a further decreased level. In short, input AC voltage variation will be amplified to produce unduly increased or decreased load current. The above operational mechanism also makes it difficult to control the inverter output energy in accordance with the varying load requirement, since variation in the load current will go back into a corresponding variation in the inverter input energy which returns an amplified variation in the load energy. Because of the above circuit limitations, the prior art inverter device fails to successfully control the inverter output energy at a desired level.